[ci] add GitHub Action workflow to label PRs based on tools they touch

[rogerluan]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I see several green ci/circleci builds in the "All checks have passed" section of my PR (connect CircleCI to GitHub if not)
• I've updated the documentation if necessary.
• I've added or updated relevant unit tests.

Motivation and Context

We keep adding these labels manually, but only when a core contributor opens/reviews a PR. A handy tool that does this automatically for us, even before we review the PR, would be helpful!

Description

This workflow will automatically label PRs with e.g. tool: fastlane_core, type: ci, tool: deliver, etc. based on the directories that are being modified in the PR.

I'm wondering if I forgot any obvious rule we should be adding? What are other files associated to the type: ci label? Are there other labels we could automate based on file patterns?

Important

This automation will only kick in when PRs are open/reopened or a new commit is pushed to it. In other words, it won't update existing open PRs. We'd need to do this in a one-off basis somehow. I intend to rebase all open PRs so that should help adding labels to all PRs that don't have merge conflicts currently, otherwise, we could merge a GHA workflow that will be trigger on cron job and then remove it once it's run once. Lmk your thoughts on this 😇

Testing Steps

Check if it adds and removes the labels as promised, in this PR. Then, switch pull_request trigger to pull_request_target, so it works with forked PRs. Read this (https://github.com/actions/labeler#notes-regarding-pull_request_target-event) to understand why this switching is needed to test.

Screenshots of tests observed in this PR:

Labels successfully added by the bot once everything was configured correctly:

Labels successfully removed when the code changed in those directories was removed:

Not sure if we should allow the bot to remove labels, see this comment of mine: #21751 (comment)

[rogerluan]

Note to reviewers: please don't merge this PR with this code in. Before merging, we need to delete pull_request and uncomment pull_request_target.

See https://github.com/actions/labeler#notes-regarding-pull_request_target-event for more details